You need to enable the SMS Module from general settings (Admin privileges required)
After configuring extension and enabling the SMS module you will see the Send SMS button in contacts block.
Click on the button to open SMS form.
Select the contact to whom you want to send. Next you can either choose from SMS templates pre-defined or you can write your own custom text in Message body field. Message type is related to type of message either promotional, transactional or other type supported via your SMS API (if applicable).
If you or your administrator have linked action type while configuring extension then it will show you the action name note while sending it.
Click on Send SMS button to send message on select contact mobile number and this will log an action if configured. (Recommended)